Iridium heading process



7, 1968 M. K. ,PARDELL 3,399,290

IRIDIUM HEADING PROCESS Filed July 255, 1965 1N VEN TOR.

ATTORNEY United States Patent 3,399,290 IRIDIUM HEADING PROCESS Milton K. Pardell, Mount Morris, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed July 23, 1965, Ser. No. 474,315 7 Claims. (Cl. 219-150) ABSTRACT OF THE DISCLOSURE A method for forging iridium metal in an electrical die presset. The iridium metal is heated to a temperature just below the recrystallization temperature of the iridium metal, for example, 1700 to 1900 F., and subjected to pressure which causes the deformation of the iridium into the desired shape.

This invention relates to the forging of metals, and more particularly to the upsetting of iridium metal wire.

Iridium metal has a high resistance to spark erosion thereby making it a very desirable metal to use as the metal in spark plug center electrodes, particularly for spark plugs requiring a center electrode which will resist spark plug erosion for long periods of time. As a result, iridium headed center electrodes are preferred over other metal electrodes in a number of aircraft spark plugs. Iridium, however, is extremely brittle and has limited ductility, thereby presenting a problem in forming these spark plug electrodes. Before this invention, upsetting iridium wire to form a headed iridium electrode resulted in a large amount of scrappage due to the brittleness and limited ductility of the iridium metal. Moreover, heating the iridium metal to a temperature sufiicient to provide the desired ductility increased the brittleness of the metal after the metal has been cooled.

It is a basic object of this invention to provide an improved method for forging iridium metal. It is another object of this invention to provide an improved method of upsetting iridium wire and rods. It is yet another object of this invention to provide a method for upsetting iridium wire and rods having a limited amount of scrappage. It is a further object of this invention to provide a method for forming headed iridium spark plug center electrodes. It is still a further object of this invention to provide a method for upsetting iridium wires and rods which yields an article less brittle than that obtained by conventional upsetting methods.

These and other objects are accomplished by a method in which the forging is performed on the iridium metal while the metal is at a temperature just below its recrystallization temperature.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawings wherein a prefered embodiment of this invention is shown.

In the drawings:

FIGURE 1 is a sectional view of the upsetting apparatus showing a two-piece die after the iridium wire has been inserted in the lower die;

FIGURE 2 is a sectional view of the upsetting apparatus showing the two-piece die after the upper die has been postiioned about the upper end of the iridium wire;

FIGURE 3 is a sectional view of that portion of the upsetting apparatus showing the position of the two-piece die after the iridium wire has been upset;

FIGURE 4 is an enlarged view of the upset iridium wire.

The invention will now be described in detail in terms of -a method of upsetting an iridium wire to form a headed center electrode for use in spark plugs and the 3,399,290 Patented Aug. 27, 1968 like. The method consists generally of placing an iridium wire of a predetermined length into the lower piece of the two-piece die and clamping the top portion of the wire with the upper die piece. As soon as the upper die piece is in a clamping relationship with the iridium wire, a transformer connected across the upper and lower die pieces admits .a current to flow through the wire to heat the wire to a predetermined temperature which is just below the recrystallization temperature of the wire. As soon as the wire is heated to the predetermined temperature, the infra-red heat from the wire causes a photocell to actuate a solenoid operated hydraulic or air mechanism which subjects the upper movable die to a downward directed impact thereby upsetting the wire to form a headed electrode. Subsequently, the ejector pin ejects the headed electrode.

Referring to FIGURE 1 of the drawings, the apparatus includes a lower die 10 having a centerbore 11 positioned longitudinally in the central portion thereof and having the same diameter as the iridium wire to be upset. The upper portion of the centerbore 11 is enlarged thereby forming the die cavity 12 having the shape of the enlarged portion of a spark plug center electrode. The lower die 10 is supported on a stationary support 14. Positioned within the die centerbore 11 and the lower support 14 is an ejector assembly consisting of an ejector pin 16 having an enlarged annular middle portion 19 which is supported on an adjustable screw-type bushing 21. The apparatus is provided with suitable means (not shown) for moving it vertically upward when it serves as 'an ejector. The bushing 21 is in interlocking threaded relationship with support 14. The apparatus includes an upper movable die 18 having a cylindrical recess 20 which has the same diameter of the iridium wire to be upset. The movable die 18 is supportedby a movable support 22 which is mounted in a manner to provide for vertical reciprocable motion. The movable support 22 is connected to a hydraulic cylinder 23 which provides the force for the vertical movement required in positioning the movable die 18 about the iridium wire 24 as well as the downward impact force required to upset the wire. A photocell 26 for measuring the temperature of the wire y sensing the infra-red heat is connected to a control panel 25 which in turn is connected to the hydraulic means 23. The dies 18 and 10 are connected electrically at points 13 and 15 in a circuit having a transformer 17 which is also connected to the control panel 25.

As shown in FIGURE 1, the iridium wire 24 is inserted in the centerbore 11 of the lower die 10 and is supported therein by the top of the ejector pin 16. The upper portion of the iridium wire 24 protrudes out of the lower die by a height greater than the depth of cylindrical recess 20 of the upper die 18. The additional height of wire is required to provide metal to fill the cavity 12, thereby forming the enlarged annular portion of the spark plug center electrode. The commercial idirium wire used in the preferred embodiment contains 99.9% pure iridium. Other iridium wires having a lower purity can also be used in this invention. The efiect of the impurities or alloying metals is to change the recrystallization temperature of the iridium containing alloy to be upset, as will hereinafter be discussed.

FIGURE 2 shows the position of the upper die 18 after it has been lowered by means of the hydraulic or air mechanism 23 so that the iridium wire 24 is received in the recess 20 thereby clamping the wire 24 snugly between the lower die 10 and the upper die 18. As soon as the dies 10 and 18 are closed to the position shown in FIGURE 2, a closed circuit is made permitting the current to pass through the wire 24 and the dies. The wire 24 having a high resistance compared to the mass of metal in dies 10 and 18 is preferentially heated to the prescribed temperature. Whenthe wire 24 has reached a temperature just below its recrystallization temperature, as will be hereinafter discussed, the photocell 26 senses the infra-red heat from the wire and transmits a single to the control panel 25. The signal does two things in the control panel 25. It shuts off the electricity going to the transformer 17 thereby stopping the heating of the wire 24 and it causes the solenoid operated hydraulic mechanism 23 to subject the upper die 18 to a downward directed impact thereby upsetting the wire 24 as shown in FIGURE 3.

The temperature at which the upsetting takes place must be below its recrystallization temperature. As is Well known, iridiums lack of ductility and its brittleness make it impractical, if not impossible, to be upset cold. Therefore, it is necessary to heat the material in order to increase its ductility to a point sufiicient for the metal to be upset. It has been found that at temperatures below 1700 F. the iridium metal will fracture instead of being deformed by upsetting methods. Iridium alloys may be formed, however, which would have sufficient ductility to be upset at a temperature lower than 1700 F. With pure iridium or iridium having 99.9% purity, a temperature of 1700 represents the lowest temperature at which iridium can be successfully upset.

The recrystallization temperature was determined on four different lots of commercial iridium wire having a purity of 99.9%. The recrystallization temperature was determined by heating samples from each iridium bath at temperatures of 1700, 1800", 1900, 2000, 2100", 2200, and 2400 F. for 20 minutes in air. The crystalline structure of the samples were micro-examined under a magnification of 150 times to determine if the iridium had recrystallized. The data obtained would give a range wherein the lower temperature indicated a temperature at which the iridium had not recrystallized and the higher temperature indicated that the iridium wire had recrystallized. It was found that the recrystallization temperature of the four different lots of commercial iridium wire having a purity of 99.9% varied between about 1900 F. to 2200 F., the individual recrystallization temperature range for each lot being 19002000 F., 2000 -2200 F., 1900-2100 F. and 2000-2l00 F. Accordingly, the recrystallization temperature of any lot of iridium wire is determined before the upsetting step is performed.

It is important that the upsetting be done at a temperature lower than the recrystallization temperature. -If the upsetting is performed at the recrystallization temperature or higher, the iridium metal becomes more brittle after it has been cooled and therefore, lacks strength and resistance to fracturing. Iridium which has been recrystallized is extremely brittle and breaks easily. This invention, therefore, involves a method of deforming iridium metal wherein the temperature employed is higher than 1700 F. in the case of pure iridium, and lower than the recrystallization temperature of the iridium metal.

The upper and lower dies remain in the position as shown in FIGURE 3 for a predetermined time, usually 0.1 to 0.2 second, in order to allow the upset iridium electrode to cool. Then the upper die 18 is returned to the position as shown in FIGURE 1. The ejector pin 16 is actuated thereby ejecting the upset iridium headed spark plug center electrode which is shown in FIGURE 4.

The specific method described pertains to the upsetting of iridium metal wire to form a headed spark plug center electrode. However, this invention applies to any forging method involving iridium metal as long as the temperature at which the iridium metal is deformed is just below the recrystallization temperature.

While the invention has been described in terms of a specific embodiment, it is understood that the scope of the invention is not limited thereby except as defined in the following claims.

I claim:

1. A method of forging iridium metal comprising the steps of heating iridium metal to a temperature just below the recrystallization temperature of said metal to increase the ductility of said metal without increasing the brittleness of said metal when cooled, and applying pressure to said metal at said temperature to deform said metal into the desired shape.

2. The method of upsetting iridium metal wire in a two-piece die comprising the steps of inserting the iridium metal wire into one of the die pieces, positioning the other die piece about the wire to clamp said wire, heating said wire to a temperature just below the recrystallization temperature of said wire to increase the ductility of said wire without increasing the brittleness of said wire when cooled, and applying a controlled pressure to said die to upset said wire into the desired shape.

3. A method as described in claim 2 wherein said temperature is between 1700 F. and a temperature just below the recrystallization temperature of said metal.

4. A method as described in claim 2 wherein said temperature is between 1700 and 1900 F.

5. The method of upsetting iridium metal wire in a two-piece die comprising the steps of inserting the iridium metal wire into one of the die pieces, positioning by hydraulic means the other die piece about the iridium metal to clamp said wire, heating said wire by an electrical current which passes through said wirc when said wire is clamped by said die to a temperature just below the recrystallization of said wire to increase the ductility of said wire without increasing the brittleness of said wire when cooled, discontinuing the heating of said wire when the photocell indicates the temperature has been reached, and upsetting said wire into the desired shape by hydraulic means actuated when said photocell indicates the temperature has been reached.

6. A method as described in claim 5 wherein said temperature is between 1700 and 1900 F.

7. The method of forming an iridium headed center electrode for use in a spark plug comprising the steps of inserting the iridium metal wire into the lower piece of a two-piece die set, positioning the upper die about the top of the wire to clamp said wire, heating said wire by means of an electric current to a temperature just below the recrystallization temperature of said wire to increase the ductility of said wire without increasing the brittleness of said wire when cooled, and applying a controlled pressure to said die to upset said wire into the shape of a spark plug center electrode having an enlarged annular portion thereon.

References Cited UNITED STATES PATENTS 1,392,553 10/1921 Bill 219- 1,692,488 11/1928 Dannell 219-l5O 1,962,647 6/1934 Maonroe 219-450 2,237,121 4/1941 Stine et al 219-149 X 2,275,763 3/1942 Howard at al. 219-l50 OTHER REFERENCES The Metals Handbook, A.S.M., 1948, pp. 1102 and 1131.

RICHARD M. WOOD, Primary Examiner. B. A. STEIN, Assistant Examiner. 

